56 



GUIDE TO THE CORAL GALLERY. 



Fig. 18.' 



Case 3, detached from the flower -like stocks of Athecate Hjdroids come 

 Upright part, ^^mjei- tj^e Anthomedusa- ; they have naked eye-spots (Fig. 4), and 



the eggs are formed in the walls of the manubrimn ; see RathTcea 

 fasckulata* a little Medusa which is given off from a small solitary 



fixed polyp with four tentacles. The early history of Pamlcm 



ronira* and Tiara pileata* both of which are Anthomediim, is 



unknown. 



The Medusae of the Thecate Hydroids come under the Leptomedusa, 



(see Ohelia* Case 3, and Model of Wwjmatodes thaJassina) ; the eggs 



are formed in the radial canals, and the sense- 

 organs are either eye-spots or " litho-cysts," 

 the latter being vesicles containing a hard 

 concretion which transmits impressions from 

 the outside to delicate " auditory " cells. In 

 certain Hydromedimc the sense-organs arise 

 on modified tentacles known as tentaculo-cysts. 

 These forms, which come under Haeckel's 

 Orders Trachomedusd' and Narcomedusce, have 

 no known " Hydroid history," and in some 

 cases are known to develop direct from the 

 Qg^ ; see Car marina hastata (Fig. 20), and 

 Model in Case 3a. Two fresh-water Medusfe 

 are known, both of which are Hydromedusif 

 Limnocodiwn sotrerUi, from the Victoria Regia 

 Tank in the Royal Botanical Society's Gardens, 

 has a shallow umbrella less than half an inch 

 in diameter and with numerous tentacles ; 

 the fixed pliase occurs as a columnar polyp 

 about a quarter of an inch in height, simple 

 or branched once or twice ; medusa buds 

 arise from the summit ; the original habitat 

 was probably the Amazon region. The second 

 species, Limnocnida tamjamjica, comes from 

 Lake Tanganyika; the umbrella is about 

 an inch across, the manubrium is very 



wide and shallow, and the stomach-cavity is nearly filled up by 



the convex lens-like central area of the under surface of the 



umbrella. 

 Case 3, Finally the Medusaj of Millepora and the swimming-bells, &c., of 



Upright part. ^^^^ Siphonophora belong to the Hydromedusan type. 



ScYPHOMEDUSAN IMedusj^. {Scyphomedusce). These include the 



Portion of Astylus show- 

 ing cyclosy stems each 

 with a central gas- 

 tropore and zone of 

 slite-like dact\ lopores. 

 (After ^Moseley ) 



